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The first big heat wave of 2025 arrived, coinciding with the official start of summer.
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Why sizzling cities are mapping hot spots street by street
Why sizzling cities are mapping hot spots street by street
The city of Reno, Nevada, is breaking records in ways it doesn't like: A 2024 analysis of 241 cities showed that Reno has heated up faster than any other city in the United States.
While the country as a whole warmed by 2.6 F on average between 1970 and 2023, Reno saw an increase of 7.6 F. A heat wave that hit Reno in July 2024 made for the hottest month in the Biggest Little City's documented history, Knowable Magazine explains.
Reno's heat is not evenly spread. In the summer, neighborhoods near the airport feel ovenlike compared to the more affluent, cottonwood-shaded homes near the Truckee River. The sidewalks of pavement-heavy shopping centers and casinos in the city center offer little relief; those in the green-space-rich neighborhoods of South Reno are kinder. The city's main thermometer, located at the airport, doesn't tell you anything about those differences.
And neither, it turns out, do temperature-mapping satellites—their resolution is far too coarse.
Which is why I found myself attaching a snorkel-like sensor to my car window on a clear and toasty August day, along with 75 other volunteers with their own sensors and cars. We split up along 20 routes and cruised through Reno in the morning, afternoon or evening. My partner and I drove a slow evening loop in the Galena neighborhood, where large, lush properties—the median home price is over $1 million—are flanked by patches of sagebrush. Even at 7 p.m., it was 92 F.
By the time we finished the loop, dusk had set in and my car thermometer had dipped to 83 F. But as we headed to the volunteer station in the city center, the gauge climbed back up to 89 F. Such gradients are common in cities, and commonly remarked upon, but they are rarely precisely measured and mapped.
Longer-lasting heat waves ahead
Today, locating the hottest parts of cities with precision is critical for guiding efforts to contend with heat's dangerous effects. As climate change brings more intense, frequent and longer-lasting heat waves, heat-related illnesses and deaths also climb. High-resolution maps can alert officials to spots facing the greatest risks, so they can plan. It's especially important when heat risk overlaps with poverty, where communities may have less access to air conditioning and fewer ways to stay cool.
Maps pieced together by the sensors "will help us be able to target, down to the street level, where we can plant more trees to help people better endure the hotter days of summer," says Brian Beffort, sustainability manager for Washoe County, home to the Reno-Sparks metro area. The maps will also guide where to focus efforts to weatherize buildings so they require less energy to cool.
Campaigns to record temperatures across city neighborhoods and create better heat maps are on the rise. Reno is one of more than 80 U.S. communities that since 2017 have completed a heat mapping project with the aid of citizen scientists, efforts overseen by the National Oceanic and Atmospheric Administration. NOAA has also supported a few international mapping efforts in cities such as Nairobi, Kenya, and Salvador, Brazil.
Local officials are using the data to plan how to adapt to, and fend off, rising urban temperatures. Some have begun to plant trees, install reflective materials and take other measures to cool the hot spots.
Targeted cooling
In many communities, heat campaign data are already being used to target spots most in need of cooling resources.
After a mapping campaign in Toledo, Ohio, officials found that the city's hot spots often overlapped with census tracts identified by the Biden administration's Climate and Economic Justice Screening Tool as lacking in commercial investment and overburdened by pollution. (In January 2025, the Trump administration removed the mapping tool and references to the project from government websites.) Some parts of the city were 11 F hotter than others at midafternoon on the summer mapping day. City sustainability coordinator Beatrice Miringu says that the data helped to secure a $6.1 million grant to plant more than 10,700 trees, funded by the 2022 Inflation Reduction Act.
In the Raleigh-Durham area of North Carolina, Cawley says that heat-map data from the area's 2021 campaign are helping officials to identify streets to spray with reflective titanium dioxide, a covering that helps to bring down street-level temperatures. They are also displayed at the Museum of Life and Science and incorporated into public school physics and history curricula.
Neighborhood organizations can also use the data to inform heat-safety outreach by, for example, calling or texting people when temperatures climb to dangerous levels.
"Heat is a hyper-local impact. It imperils people really differently on a neighborhood-by-neighborhood, and sometimes block-by-block, level," says Cawley. "People who are impacted ought to have a pretty big say in how this is mitigated and dealt with." Some neighborhoods, for example, might need immediate help paying for increased air conditioning use; after all, trees need years to grow into a shady canopy.
Often, the most effective way to expand canopy is to maintain the health of existing trees rather than only planting new ones—though such maintenance efforts are more expensive and get less press, says Erica Smith Fichman, city forester for Philadelphia's Parks and Recreation Department, where officials use heat mapping data to design the city's forestry plans.
Data from Reno and elsewhere will also help to improve local climate models and so better predict warming impacts down to the street level, says John Mejia, a climatologist with Nevada's Desert Research Institute. Mejia led a similar mapping effort in Houston and advised another in Las Vegas. He has built an urban climate model of Las Vegas, which he can use to "play God" and, for example, add more trees to the simulation to observe their effect on temperatures. "We can use the heat map data to contrast the model output versus the observations," he adds, thus improving the model.
